Almost all new vaccines are less effective than required, particularly when infection has already established itself. The best vaccines are based on empirical observations. These are e.g. killed or attenuated micro-organisms. In general such vaccines comprise all components required for the induction of effective and protective immune responses. However, these vaccines are usually poorly defined. Moreover, they contain many unknown components, the mechanism of action is usually largely unknown, they can't be produced reproducible and they thus don't fulfil the criteria for modern drugs. They are only applied because nothing better is available at present. In the case of vaccines against cancer things are even more complicated, because the utilization of material from nature, i.e. cancer cells, is unacceptable.
In order for vaccines to elicit a protective immune response against microbial pathogens a sufficient efficacy of vaccine uptake by antigen presenting cells (APC) such as dendritic cells and macrophages is required. The APC have to internalize the antigens and to transport them to the regional draining lymph nodes where they process and present the antigenic peptides on class I and class II MHC molecules for the activation of CD8+ and CD4+ T cells, respectively (Zinkernagel et al. (1999) Immunol Rev 156:199-209; Banchereau and Steinman (1998) Nature 392:245-252).
However, due to poor uptake by APC, several vaccines have limited immunogenicity, even though they are comprised of proteins that are antigenic in humans. The poor uptake is probably because many vaccinating proteins in solution, as well as particulate vaccines, lack markers that identify them for internalization and processing by APC and therefore the uptake of such a vaccine is mediated only by random endocytosis into APC and thereby minimal (Abdel-Motal et al. (2009) Vaccine 27:3072-3082).
The immunogenicity of microbial vaccines, such as tetanus toxoid, was shown to be improved when the vaccines are administered as immune complex with their corresponding IgG antibody (Raveth and Clynes (1998) Annu Rev Immunol 16:421-432; Schuurhuis et al. (2002) J Immunol 168:2240-2246; Gosselin et al. (1992) J Immunol 149:3477-3481). However, a disadvantage is that the immune complex has to be formed with an antibody that has an Fc domain which effectively binds to the Fc-gamma receptor on APC, whereas the Fc domain of many monoclonal antibodies has poor interaction with the Fc-gamma receptor on APC. A further disadvantage is that in the complex immunodominant peptide epitopes may be masked, and thus result in poor immunogenicity.
Another strategy to improve the immunogenicity of vaccines is presented in WO 2008/118487, wherein an influenza virus bearing α-gal epitopes (Galα1-3Galβ1-4(3)GlcNAc-R) is disclosed, resulting in enhanced targeting of the virions to APC and in a heightened humoral and cellular immune response to influenza. However, synthetising α-gal epitopes is more difficult as compared to synthesis of regular peptides and as a consequence more expensive.
Volk et al. (1984, Infect. Immun. 45:604-609) disclose a tetanus toxin fragment comprising the N-terminal half of the toxin heavy chain including the sequence GITELKKL that spans residues 383 to 390 of the toxin heavy chain as depicted in SEQ ID NO:3 herein and antibodies thereto.
Raju et al. (1996, J. Autoimmun. 9:79-88) disclose overlapping synthetic peptides, each 20 residues in length, used to determine the epitope repertoire of human CD4+ T cells. One of the peptides recognized by said cells spans residues 371-390 of the tetanus toxin heavy chain, fully comprising the sequence GITELKKL (residues 383-390 of SEQ ID NO: 3).
Demotz et al. (1989, J. Immunol. 142:394-402) disclose monoclonal antibodies that bind B fragment of tetanus toxin and recombinant fragments 744-1315 and 604-1315 comprising the sequence GITELKKL (residues 383-390 of SEQ ID NO: 3).
Engstrom et al. (J. Immunoassays 16: 231-245) disclose a “peptide 20”, comprising the sequence GITEL (SEQ ID NO: 220), that is recognized by human IgG and IgA antibodies.
WO 2004/000873 discloses a conjugate comprising a tetanus epitope derived from the tetanus heavy chain sequence 830-843 comprising the sequence GITE (SEQ ID NO: 221).
Fischer et al. (1994, Mol. Immunol. 31:1141-1148) disclose epitope mapping of antibodies raised in mice and rabbits to tetanus toxoid and that hexapeptides from the region spanning residues 350-400 of the tetanus toxoid heavy chain show high reactivity to the antibodies.
However, none of the prior art teaches or suggests peptides comprising a linear epitope from tetanus toxin against which circulating antibodies are frequently present in the human population. Nor does the prior art teach or suggest the use of these peptides for conjugation to an antigen against which an immune response is desired, in order to increase the efficiency with which this desired immune response can be induced.
Thus, there is still a need in the art for methods and means to increase the immunogenicity of vaccines.